In long distance and large capacity optical communication systems, there has been a problem with the waveform distortion of optical signals due to the chromatic dispersion of an optical transmission line and so on. In order to deal with this problem, pre-equalization at the transmitting end of an optical communication system has been proposed for preliminarily forming the waveform of signals so as to compensate for the characteristics of the optical transmission line.
Moreover, polarization multiplex communication in order to achieve high-speed optical communication systems has received attention. The polarization multiplex communication is capable of transmitting two optical signals by using a pair of mutually orthogonal polarization, and thus it is possible to double the transmission capacity.
The polarization characteristics of an optical transmission line are not constant in optical communication systems, and the fluctuation in the polarization characteristics causes a degradation in communication quality. For example, polarization mode dispersion (PMD), polarization dependent loss (PDL), and polarization dependent gain (PDG) may vary with respect to time. For this problem, methods for reducing the polarization-dependent degradation such as PMD/PDL/PDG, which scramble the polarization for transmitting signals, have been proposed.
The above-mentioned pre-equalization methods and polarization scrambling methods are realized by using digital signal processing. In such cases, an optical transmitter includes a digital signal processor and an optical modulator. The digital signal processor generates a drive signal from input data via the signal processing including pre-equalization and/or polarization scrambling. Then, the optical modulator uses the generated drive signal to generate an optical signal which transmits data.
As a related art, optical transmission systems for improving the quality of a transmission signal by using the digital signal processing have been proposed. An optical transmitter of such optical transmission systems includes an optical modulator to output a modulated optical signal, and a signal processor at the transmitting end to perform digital signal processing at the transmitting end on an input signal so as to apply polarization change to the modulated optical signal. The optical modulator performs the optical modulation according to the input signal on which the digital signal processing at the transmitting end is performed by the signal processor at the transmitting end. The optical receiver includes a converter to convert the optical signal from the optical transmitter via a transmission line into a digital electric signal for each polarization component, and a polarization signal processor at the receiving end to perform the digital signal processing at the receiving end on the digital electric signal from the converter, where the polarization change of substantially inverse characteristics with respect to the polarization change at the signal processor at the transmitting end is applied to the received signal. (For example, see Japanese Laid-open Patent Publication No. 2010-109705.)
As described above, the digital signal processing is capable of applying a desired state or desired characteristics to drive signals of the optical modulator, and thereby it is possible to generate an optical signal having a desired state or desired characteristics. However, when several states or characteristics are applied to drive signals by using the digital signal processing, the amplitude of the drive signals may deviate from the drive amplitude with which the optical modulator operates in an optimal state. When the optical modulator operates with drive signals of an inappropriate amplitude, the quality of a transmission signal deteriorates.